Flake8: Fix and enable H405

Fix H405:
H405  multi line docstring summary not separated with an empty line

With this patch, flake8 passes all checks (incl. hacking checks)
except the on-purpose disabled check H904.
Closes-Bug: #1333290

Change-Id: If41259aefc4d6d588da5036f1f25df17b88db04f
Merge "Flake8: Fix and enable H405"